The present invention is in the field of lithographic printing, and relates in particular to the steps for preparing imageable or imaged printing plates for mounting on a plate cylinder of a printing press.
Common practice in the newspaper printing industry includes the steps of radiation imaging a photosensitive coating on a plate as supplied by the plate manufacturer, developing the plate by chemically reacting the imaged coating with an aqueous fluid, and bending each end (head and tail) of the developed plate before delivering the plate to the press where the plate is mounted on the plate cylinder of the press. The imaging step includes at least three reference marks in the margins of the plate, at corners. Developing reveals at least the reference marks as well as the main imaged areas of the plate.
The acute bends in the head and tail of the plate allow for the accurate attachment of the plate to the printing press cylinder. These acute bends slip into grippers that tightly hold the plate to the cylinder when the press is in operation. The accuracy of the head and tail plate bends is critical to within thousandths of an inch. Automated vision bending equipment uses cameras to define the exact location of each bend. These cameras detect the reference marks on the head and tail of the plate.
This way of revealing reference marks is not compatible with the growing interest in so called “on-press” development, whereby the step of developing the plate in a fluid before mounting the plate on press is avoided because the plate is developed by dissolution or dispersion in the fountain fluid while it is on the plate cylinder of the press. Instead, the coating includes a dye component that discolors when imaged, thereby rendering the reference marks and all the imaged areas of the coating somewhat visible before the plate reaches the bending tool. This discoloration has low contrast relative to the unimaged coating around it, resulting in degradation in positional detection and thus in the accuracy of the bends. Although special cameras are being designed to accommodate the lower contrast reference marks, success has so far been limited.